If I run the Xtend standalone compiler in the headless build (with
Buckminster), the corresponding .java._trace files contain a reference
to the original xtend file with an absolute path (starting with
file:/home/...). This happens in spite of the compiler being called
with command line argument: "-d xtend-gen src".

This way, the corresponding generated source bundles are not very
useful, since you can't inspect the source of a Java class generated
from an xtend file (Eclipse will show an empty file in the editor). Of
course you can manually open the corresponding .xtend file in the source
bundle but that's not convenient.

This does not happen if the xtend compilation takes place in the
workbench: the .java._trace files contain a reference to the original
xtend file with a relative path, and the corresponding source bundles
can be efficiently used.